All-trans retinoic acid potentiates Taxotere-induced cell death mediated by Jun N-terminal kinase in breast cancer cells

Taxotere is a cytotoxin effective in treating breast and prostate cancer. It stabilizes microtubules and causes catastrophic cell cycle arrest in G2/M. Taxanes also initiate apoptosis by activating signal pathways, such as the jun N-terminal kinase (JNK) pathway. Strategies aimed at potentiating cell death signaling may improve their efficacy while lessening the potential side effects. We reported that all-trans retinoic acid (ATRA) potentiated taxane-mediated cell death. Here we investigated whether ATRA potentiates cell death signaling through the JNK pathway. Activation of JNK by Taxotere 0.01, 0.1 and 1.0 muM was observed at 24h in adherent cells and increased at 48h. Taxotere 0.001 muM-induced JNK activation started after 48h and increased at 72h. The timing and intensity of PARP cleavage was similar to that of JNK activation. JNK activation and PARP cleavage induced by 30 nM Taxotere at 48h were reversed by curcumin, PD169316 and SP600125, JNK inhibitors in order of progressive specificity. None of these inhibitors had an effect on p38 or ERK phosphorylation. All three inhibitors reversed Taxotere-induced phosphorylation of Bcl-2. ATRA induced JNK activation at 24, 48 and 72h. Incubating cells with ATRA 0.01 muM for 3 days prior to Taxotere treatment potentiated Taxotere-induced JNK activation 24 and 48h later, an effect sustained for 72h. Cytotoxicities from 3-day ATRA 0.01 muM incubations were synergistic with subsequent 1-h Taxotere 0.01, 0.1 and 1.0 muM incubations in breast cancer cell lines MCF-7 and MDA-MB-231 and in prostate cancer cell lines LNCaP and PC-3, and additive in breast cancer cell line SK-Br-3. These data demonstrate the potentiation of Taxotere-induced cell death by ATRA pretreatment in breast and prostate cancer cells, and support a mechanism through accentuated and sustained JNK activation.